The present invention relates to circuit protection apparatus and more particularly to a novel solid state tripping circuit for controlling the operation of a circuit breaker.
A conventional circuit breaker may utilize a combination of electromagnetic and electrothermal mechanisms to initiate the mechanical separation of its current carrying contacts in the event of an excessive flow of current in the circuit protected by the circuit breaker.
A bimetallic element may thermoelectrically monitor the value of current flow for actuation of the mechanical contact separation mechanism after a long time delay established by the inverse current-squared-versus time curve of the bimetallic element for circuit currents at least a selected percentage, e.g. 10%, greater than the rated current of the circuit breaker. A single long time delay curve is permanently established for a particular configuration of bimetallic element and cannot be easily varied to accommodate different values of rate current, initial and final points on the curve or rate of curve change. Disassembly of the circuit breaker and substitution of a different bimetal element is normally required to change the long time delay response to the conventional circuit breaker.
An electromagnetic trip coil may actuate the mechanical contact separation mechanism after a comparatively short time delay for overcurrent ratios greater than a range of ratios of the bimetal element. The time delay curve of the electromagnetic trip mechanism is a function of the number of conductor turns utilized and is inversely proportional to the magnitude of current flow through the coil. The curve is generally not adjustable without disassembly and replacement of selected parts of the circuit breaker unit.
Thus, the proper operation of the inter-related mechanical elements of a circuit breaker utilizing thermoelectric and electromagnetic trip mechanisms requires relatively precise and unadjustable parts and results in a relatively costly unit. The normal current rating of a particular conventional circuit breaker is not easily varied and it is not possible to test the function of the conventional circuit breaker unit without requiring disconnection of the line and load connectors therefrom.
It is desirable to provide a circuit breaker with a contact separation mechanism actuated by a tripping circuit having a maximum proportion of solid state components to realize a high degree of efficiency, reliability and economy while allowing the tripping circuit to be easily tested without removing line and load conductors from the circuit breaker. It is also desirable to provide a tripping circuit which will allow a full range of adjustment in the various time delays required for a range of over-current ratios, as well as enabling the adjustment of the normal current rating of the circuit breaker unit over a relatively wide range with a minimum number of component adjustments.